I used one for my daughters Walmart MTB. Bike is women's style with a step through frame. Pretty sure I used a Yakima one. Didn't work so well. It's supposed to clamp to the seatpost and the steerer tube to make a top tube that can be held by a trunk style rack. Problem was that it didn't grip either location tight enough so there was lots of movement. If you can get one that will grip tight it might be better.
My solution was a different style of rack - platforms are the best. They even make them for trunk mounts now.

https://www.thule.com/...form-pro-2-_-1685475

Thanks for the feedback, I saw several reviews saying the movement was an issue.

Yeah the platform rack is the way to go, but the price isn't here. I currently have the Apex swing which is great for the road bikes, but I sort of need to work the mtn bikes on there. Someone near me is selling one for $15 so I might give it a try.